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more about securing an office

My post about securing an office received many comments, so many that I had to write another blog entry to respond to them and also add some other things I didn’t think of before.

One suggestion was to use pam_usb to store passwords on a USB device. It sounds like it’s worth considering, but really we need public key encryption. I don’t want to have a USB device full of keys, I want a USB device that runs GPG and can decrypt data on demand – the data it decrypts could be a key to unlock an entire filesystem. One thing to note is that USB 2.0 has a bandwidth of 30MB/s while the IDE hard drive in my Thinkpad can sustain 38MB/s reads (at the start – it would be slower near the end). This means that I would approximately halve the throughput on large IOs by sending all the data to a USB device for encryption or decryption. Given that such bulk IO is rare this is feasible. There are a number of devices on the market that support public-key encryption, I would be surprised if any of them can deliver the performance required to encrypt all the data on a hard drive. But this will happen eventually.

Bill made a really good point about firewire. I had considered mentioning it in my post but refrained due to a lack of knowledge of the technology (it’s something that I would disable on my own machines but in the past I couldn’t recommend that others disable without more information). Could someone please describe precisely which 1394 (AKA Firewire) modules should be disabled for a secure system? If you don’t need Firewire then it’s probably best to just disable it entirely.

To enable encryption in Fedora Core 6 you need something like the following in /etc/crypttab:

home_crypt /dev/hdaX /path/to/key
swap /dev/hdaX /dev/random swap

Debian uses the same format for /etc/crypttab.

The Peregrine blog entry in response to my entry made some really good points. I wasn’t aware of what SUSE had done as I haven’t done much with SUSE in the past. I’m currently being paid to do some SUSE work so I will learn more about what SUSE offers, but given the SUSE/MS deal I’m unlikely to use it when I don’t have to. Before anyone asks, I don’t work for SUSE and given what they have just done I will have to reject any offer of employment that might come from them.

I had forgotten about rsh and telnet. Surely those protocols are dead now? I use telnet as a convenient TCP server test tool (netcat isn’t installed on all machines) and never use rsh. But Lamont was correct to mention them as there may be some people still doing such things.

The Peregrine blog made an interesting point about Kerberised NFS being encrypted, I wasn’t aware of this and I will have to investigate it. I haven’t used Kerberos in the past because most networks I work on don’t have a central list of accounts, and often there is no one trusted host.

I strongly disagree with the comment about iSCSI and AoE “Neither protocol provides security mechanisms, which is a good thing. If they did, the additional overhead would affect their performance“. Lack of security mechanisms allows replay attacks. For example if an attacker compromises a non-root account on a machine that uses such a protocol for it’s root filesystem, the victim might change their password but the attacker could change the data back to it’s original values even it if was encrypted. Encryption needs to have sequence numbers embedded to be effective, this is well known – the current dmcrypt code (used by cryptsetup) encrypts each block with the block ID number so that blocks can not be re-arranged by someone who can’t decrypt them (a weakness of some earlier disk encryption systems). When block encryption is extended to a network storage system I believe that the block ID number needs to be used as well as a sequence ID number to prevent reordering of requests. CPU performance has been increasing more rapidly than hard drive performance for a long time. Some fairly expensive SAN hardware is limited to 40MB/s (I won’t name the vendor here but please note that it’s not a company that I have worked for), while there is faster SAN hardware out there I think it’s reasonable to consider 40MB/s as adequate IO performance. A quick test indicates that the 1.7GHz Pentium-M CPU in my Thinkpad can decrypt data at a rate of 23MB/s. So to get reasonable speed with encryption from a SAN you might require a CPU which is twice as fast as in my Thinkpad for every client (which means most desktop machines sold for the last two years and probably all new laptops now other than the OLPC machine). You would also require a significant amount of CPU power at the server if multiple clients were to sustain such speeds. This might be justification for making encryption optional or for having faster (and therefore less effective) algorithms as an option.

I believe that the lack of built-in security in the AoE and iSCSI protocols gives a significant weakness to the security of the system which can’t be fully addressed. The CPU requirements for such encryption can be met with current hardware even when using a strong algorithm such as AES. There are iSCSI accellerator cards being developed, such cards could also have built in encryption if there was a standard algorithm. This would allow good performance on both the client and the server without requiring the main CPU.

Finally the Peregrine blog entry recommended Counterpane. Bruce Schneier is possibly the most widely respected computer security expert. Everything he does is good. I didn’t mention his company in my previous post because it was aimed at people who are on a strict budget. I didn’t bother mentioning any item that requires much money, and I don’t expect Counterpane to be cheap.

Simon noted that developing a clear threat model is the first step. This is absolutely correct, however most organizations don’t have any real idea. When advising such organizations I usually just invent a few possible ways that someone with the same resources and knowledge as I might attack them and ask whether such threats seem reasonable, generally they agree that such things should be prevented and I give further advice based on that. It’s not ideal, but advising clients who don’t know what they want will never give an ideal result.

One thing that I forgot to mention is the fact that good security relies on something you have as well as something you know. For logging in it’s ideal to use a hardware security token. RSA sells tokens that display a pseudo-random number every minute, the server knows the algorithm used to generate the numbers and can verify that the number entered was generated in the last minute or two. Such tokens are sold at low prices to large corporations (I can’t quote prices, but one of my clients had prices that made them affordable for securing home networks), I will have to discover what their prices are to small companies and individuals (I have applied to evaluate the RSA hardware). Another option is a GPG smart-card, I already have a GPG card and just need to get a reader (this has been on my to-do list for a while). The GPG card has the advantage of being based on free software.

One thing I have believed for some time is that Debian should issue such tokens to all developers, I’m sure that purchasing ~1200 tokens would get a good price for Debian and the security benefits are worth it. The use of such tokens might have prevented the Debian server crack of 2003 or the Debian server crack of 2006. The Free Software Foundation Fellowship of Europe issues GPG cards to it’s members, incidentally the FSFE is a worthy organisation that I am considering joining.